1. Technical Field
The present invention relates to the general field of flipchip light emitting diodes and, more particularly, to reducing the far-field variations in the intensity of radiation emitted from such devices.
2. Description of Related Art
Light emitting diodes (xe2x80x9cLEDsxe2x80x9d) are a highly durable solid state source of light capable of achieving high brightness and having numerous applications including displays, illuminators, indicators, printers, and optical disk readers among others. LEDs are fabricated in several geometric configurations, including a xe2x80x9cflipchipxe2x80x9d design having a reflective ohmic contact and a second ohmic contact on one face and a substrate transmissive to the LED""s emitted light as the opposite face. The ohmic contact having opposite polarity from the reflective contact is located so as not to interfere significantly with the light exiting from the LED. Typically, the reflective ohmic contact is positive and the other contact is negative since the diffusion length of holes (positive charge carriers) in typical LED materials is shorter than the diffusion length of electrons (negative charge carriers). Thus, the less conductive p-layer needs to cover a large area of the LED. The long diffusion length of electrons makes it practical to inject electrons over a restricted surface area by means of a relatively small contact.
Some configurations of flipchip LEDs have the light emitting active region located in close proximity to the reflective contact. In particular, interference patterns occur if the separation of the active region from the reflective contact is less than approximately 50% of the coherence length of the light emitted by the active region. In such cases, light from the active region directly exiting from the LED creates interference patterns with light from the active region exiting from the LED following reflection by the reflective contact. These interference patterns cause spatial variations in the intensity of the light emitted from the LED, particularly in the far-field region.
Far-field intensity variations are detrimental to the performance of a single LED, but a potentially more serious problem is the variation of far-field intensity from LED to LED due to interference patterns. In general, the position, intensity and structure of the interference maxima and minima are functions of the location, reflective properties, planarity and other characteristics of the surfaces and materials the light encounters on its journey from the active region to the far-field region. In particular, modest and random variations in geometry from one LED to another are inherent in any practical LED manufacturing process. Such geometric variations readily cause sizable and largely unpredictable variations in the interference patterns and, hence, in the far-field radiation intensity. Reducing these intensity variations is one objective of the present invention.
The present invention relates to texturing the reflective contact of the LED so as to create at least two distinct interference patterns such that intensity variations caused by the interference patterns tend to compensate intensity minima with intensity maxima. Appropriate texturing results in more uniform patterns of radiation emitted by the LED and a reduction in the variations in far-field intensity from LED to LED.
The present invention relates to a flipchip LED comprising a light emitting active region having a reflective ohmic contact separated from the active region by one or more layers. Light emitted from electron-hole recombinations occurring in the active region and directly exiting from the LED creates interference patterns with light exiting from the LED following reflection from the reflective contact. The structure of the interference patterns is determined by the structure and materials of the LED, including the spacing from the active region to the reflective contact that is subject to largely unpredictable variation from one LED to another during fabrication. Inconsistent far-field radiation patterns are the undesirable result. The present invention reduces the spatial variation of light emitted from LEDs by introducing appropriate texture into the surface of the reflective layer. At least two reflective planes are provided in the reflective contact parallel to the light emitting region such that a stable interference pattern occurs in the far-field radiation. The set of reflective planes includes at least two planes separated by an odd integral multiple of (xcexn/4) where xcexn is the wavelength of the light in the layer between the reflecting planes and the active region, resulting in compensating interference maxima and minima, and a more consistent far-field radiation pattern.